Sealing a 35 watt vehicle HID lamp is difficult. The lamp is small, and the operating pressure may reach 60 atmospheres. The operating temperature of the press seals is considerably higher than that of a conventional metal halide lamp. It is known that thoriated tungsten can seal better with the quartz lamp envelope than can un-thoriated tungsten, and thoriated tungsten similarly has better electron emission characteristics. However, the thorium in the anode has been found to be transferred to the cathode during lamp operation. The transfer of thorium to the cathode causes an ever-changing thorium distribution on the cathode. The arc then wanders from place to place on the cathode as it seeks the point with the most emissive point. The wandering arc makes use of a thoriated electrode in an optical situation, such as a light source in a headlamp, difficult or unacceptable. There is then a need to improve lamp seals without using thoriated electrodes.
Normally, during cooling or lamp operation, the internal rod along its length cracks away from the envelope body, leaving a small crack between the internal rod and the envelope body. Fill materials can then migrate in the crack along the rod surface to where the rod and foil are welded. This process can be enhanced by electrical or mechanical pumping of the fill materials into the leading edge of the crack. The fill materials are then withdrawn from the internal lamp process. The fill materials can also react with the foil, the rod and the quartz to form compounds with still different thermal expansion characteristics. These materials can slowly leverage the crack to open farther, and by inching along in this fashion can cause the lamp seal and therefore the lamp to fail. Electric pumping along the rod surface acts to continually resupply materials for these reactions. Also the nonuniform adhesion to the internal rod can cause cracks which propagate from the initial condition along the internal rod to extend to the surface of the quartz press. These cracks then cause loss of hermeticity and lamp failure. There is then a need to block electrode crack extension and the resulting lamp failures.
Thoriated electrodes allowed a straight cleaving between the inner electrode and the adjacent quartz that deterred cracks from spreading to the surface resulting in lamps that leaked. So electing to use un-thoriated or bonding type tungsten electrodes, that were then normally processed, yielded cracks along the inner electrode rod that spread toward the surface. FIG. 1 shows a cross sectional view of a prior art high intensity discharge lamp made with a bonding type tungsten showing the cracking around the inner electrode rod. The lamp is a miniature HID lamp typical of an automotive HID lamp. FIG. 2 shows an artist's rendering of the cracking area of FIG. 1. The cracks are shown to spread away from the inner electrode rod in a fashion that leads to extension and then connection to the exterior surface. An unacceptable number of these lamps would go on to leak and fail. There was then a need for an HID lamp with a bonding type tungsten inner electrode with a seal that did not crack so as to leak. This is particularly true for very high pressure lamps, and lamps with very high thermal gradients cross their seals. Both of these factors exist in miniature HID lamps.